Due to environmental concerns and newly enacted rules and regulations, petroleum products are expected to meet lower and lower limits on contaminates, such as sulfur and nitrogen. New regulations require the removal of sulfur compositions from liquid hydrocarbons, such as those used in gasolines, diesel fuels, and other transportation fuels. For example, ultra low sulfur diesel (ULSD) requirements are typically less than about 10 ppm sulfur, and current regulations permit up to about 30 ppm sulfur in gasoline. A reduction in sulfur levels to less than about 10 ppm in gasoline fuels also may be desirable.
Hydrodesulfurization is a hydrotreating process often used for removal of sulfur from olefinic naphtha streams by converting sulfur in the feed to hydrogen sulfide via contact with suitable catalysts. In some cases, high temperatures and pressures may be required to obtain the desired low levels of sulfur. High temperature processing of olefinic naphtha, however, may result in a lower grade fuel due to saturation of olefins leading to an octane loss. Low octane gasoline may require additional refining, isomerization, blending, and the like to produce higher quality fuels suitable for use in gasoline products. Such extra processing adds additional cost, expense, and complexity to the process, and may result in other undesirable changes in the products.
Because olefin saturation is generally favored at higher reaction temperatures, one form of hydrodesulfurization employs relatively mild temperatures in a hydrotreating reaction zone to favor desulfurization reactions relative to reactions resulting in olefin saturation. At such conditions, however, hydrogen sulfide produced during the hydrotreatment stage frequently reacts at these relatively mild conditions to form mercaptans. These reactions are often called reversion or recombination reactions.
The presence of recombined sulfur in olefinic naphtha streams may render it more difficult to achieve desirable low sulfur levels. In some cases, the recombination of sulfur can be prevented by saturating the olefins, but as discussed above, olefin saturation in naphtha results in an undesired octane loss. In other cases, recombined sulfur can be removed using various methods such as aqueous treatment methods, base solutions, phase transfer catalysts to suggest but a few. Such additional processing, however, adds cost and expense to the refiner. Moreover, the recombined mercaptans can be branched or have high molecular weights rendering them more difficult to completely remove from hydrocarbon streams.
In some cases, mercaptan formation in naphtha desulfurization can be minimized using a two-stage hydroprocessing unit with or without interstage removal of hydrogen sulfide. For example, in a first stage, a hydroprocessing reaction zone removes a large portion of the sulfur from the hydrocarbon stream to form hydrogen sulfide. The effluent from the first stage reaction zone then may be cooled and the hydrogen sulfide removed prior to a second stage reactor. The liquid effluent without hydrogen sulfide is then reheated and fed to a second stage reaction zone where another hydroprocessing zone removes the remaining sulfur to desired levels. In other cases, the effluent from the first stage is sent directly to the second stage.
Separating the hydrogen sulfide from the effluent prior to the second reaction zone generally minimizes mercaptan formation in the second reaction zone because there is minimal hydrogen sulfide to recombine. In many cases, the second stage reactor is operated in the same temperature range as the first stage reactor to disfavor olefin saturation. Therefore, if the hydrogen sulfide is not removed prior to this second reaction zone, sulfur recombination would most likely occur at such lower second stage temperatures. Interstage removal of hydrogen sulfide, however, adds complexity and cost to the refining process.
Catalysts and coated catalysts to desulfurize hydrocarbon streams, such as crude oils, heavy oils, vacuum gas oils, naphtha, and other gasoline boiling hydrocarbon streams often include bismuth, molybdenum, vanadium, tungsten, cobalt, nickel, palladium, platinum, iron, and mixtures thereof to remove sulfur from the hydrogen streams. Common operating conditions range from about 200° C. (392° F.) up to about 600° C. (1,112° F.). However, as discussed above, when desulfurizing olefinic naphtha or other gasoline boiling hydrocarbons at high temperatures, such as in some cases above about 315° C. (600° F.), the catalysts also concurrently saturate olefins leading to a loss of octane. As discussed above, decreasing temperatures to minimize olefin saturation would then tend to favor mercaptan formation.
Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial petroleum hydrocarbon conversion processes, there is always a demand for new methods and flow schemes. In many cases, even minor variations in process flows or operating conditions can have significant effects on both quality and product selection, as well as on economic considerations, such as capital expenditures and operational utility costs.